A typical septum, such as stopper used to seal a vial or other device, is made of a material with some elongation property, such as an elastomer, or a rubber-like material, such as thermoplastic elastomer or silicone. The septum comprises an exterior surface defining a penetration zone that is penetrable by a needle, and an interior flange located on the underside of the septum that is received within, or otherwise attached to seal an opening in the vial or other type of device. The septum typically defines a flat exterior surface and a certain wall thickness in and around the penetration zone.
One of the drawbacks of such prior art septa is that when a needle penetrates through the penetration zone, the exterior surface that engages the needle cracks at the penetration site and can, in turn, invaginate, curl or roll inwardly at the opening about the needle, particularly under the axially-inward force exerted on the septum by the needle. As a result, any germs, bacteria or other contaminants on the exterior surface at the needle opening can be placed in communication with, and contaminate the interior of the vial or other device.